Up to this time, an apparatus as illustrated in FIG. 1 has been used as a focus error detector. In the illustration, a light source 1 transmits light that passes through a focusing lens 2. A half-mirror 3 is provided in the path of the light beam that passes through the focusing lens 2. An objective lens 4 causes light that is not reflected from the half-mirror 3 to be condensed as a spotlight with a very small diameter and to impinge upon an information storage surface 5a of a storage medium 5.
A part of the light is reflected by the information storage surface 5a of the storage medium 5 and passes back through the objective lens 4 and is reflected by the half-mirror 3 to a condenser 6. A cylindrical lens 7 gives an astigmatism to the reflected light passing through the condenser 6. Between two focus lines produced by non-point light fluxes passing through the cylindrical lens 7, that is, between the focus line fs in a sagittal direction and the focus line fm in a meridional direction, there exists a position where the light flux has a circular cross section. A light detector 8 is placed so that a light-receiving surface 8a is located at this position.
The light detector 8 is a device known as a quadrant-separated type detector consisting of four independent elements 8.sub.1 through 8.sub.4 arranged closely spaced but separate as shown in FIG. 2. Two perpendicularly intersecting straight lines form boundary lines. The light fluxes having an astigmatism enter this light detector 8 so that each of the focus lines fs and fm is in the direction shown in FIG. 2. In this case, the focus line fs causes the elements 8.sub.1 and 8.sub.3 to receive the maximum quantity of light and causes the elements 8.sub.2 and 8.sub.4 to receive the minimum quantity of light. For the focus line fm, the situation is the opposite.
The output signals from the elements 8.sub.1 and 8.sub.3 are added by the adder 9 while the output signals from the elements 8.sub.2 and 8.sub.4 are added by the adder 10. The sum-signals obtained from the adders 9 and 10 are subtracted by the subtracter 11. A difference-signal from the subtracter 11 is used as a focus error signal and focus control in the optical system shown in FIG. 1 is performed according to this focus error signal.
When the information storage surface 5a of the storage medium 5 is at a focusing position for the illumination beam, the reflected light fluxes are evenly distributed on the light-receiving surface 8a of the light detector 8 as shown graphically by oblique lines in FIG. 3(b). Therefore, the focus error signal FE is expressed as follows, where A.sub.1 and A.sub.2 represent the output signals of the elements 8.sub.1 and 8.sub.3, respectively, and B.sub.1 and 8.sub.3 represent those of the elements 8.sub.2 and 8.sub.4, respectively: EQU FE=(A.sub.1 +A.sub.2)-(B.sub.1 +B.sub.2)=0
On the contrary, if the above information storage surface is not at the focusing position, a distribution of the reflected fluxes will be as shown by oblique lines in FIGS. 3(a) and FIG. 3(c) resulting in FE&gt;O of FE&lt;O. As shown in FIG. 4, the focus error signal FE can be represented by an S curve. The sign and level of the signal FE changes are represented as a consequence of the direction and amount of a change in the distance between the illumination beam focusing point and the information storage surface 5a. The direction and the amount of focus offset are obtainable from the sign and level of the focus error signal FE.
In conventional detectors having the above explained structure, there is a defect which causes a focus drive mechanism servo system to be disengaged when a heavy shock or the like is applied to the system. The rate of increase in the focus error signal FE becomes small if the amount of focus offset becomes large to such an extent that the reflected light fluxes may overflow the light-receiving surface 8a of the light detector 8. The output of the detector becomes unreliable.